@khaase great to hear – do you have a pn for the cap – be interested see what the internal ESR is.
Seems to me a way to monitor the battery for maximum life is to read the voltage unloaded, and then again when it is loaded. The capacitor moderates the voltage reading, but still droops. So it would be how much droop (lack of charging from high impedance battery) can tell you something, and how much accuracy do you need from the ADC to be able to effectively detect a significant droop.
BTW here are some observation on the battery voltage measurement.
https://github.com/EnviroDIY/EnviroDIY_Mayfly_Logger/issues/36 ~ I implemented an algorithm to reduce the noise.
https://github.com/EnviroDIY/EnviroDIY_Mayfly_Logger/issues/32 ~ discussion across different Mayfly versions
and if you have anything to add to this https://github.com/EnviroDIY/EnviroDIY_Mayfly_Logger/issues/29
Some real world, with two monitoring sources, one a resistor divider to 12bit ADC, and the other the internal method, https://monitormywatershed.org/sites/TUCA-Na13/ –
https://monitormywatershed.org/tsv/TUCA_PO03/ – battery V with inter Vbat and external STC3100 to read battery V & Est battery capacity – however turns out to be hard problem to calculate